1. Field of the Invention
This invention relates to the field of speakers, particularly but not exclusively to a dual diaphragm piezo-electric speaker for an integrated hands-free portable communications device.
2. Description of the Prior Art
Integrated hands free (II-IF) handsets are a relatively recent development in mobile telephone technology. As such handsets become lighter and more compact, there is an ever-increasing requirement for the size and weight of speakers to be reduced and for quality to be increased. This is especially so as speakers are used for polyphonic ringer melodies, downloaded midi music files, text-to-speech conversion, FM radio and so on. Efficiency is also an issue when trying to maximize talk time with IHF speech.
Many different types of speaker are known, including a single diaphragm gas filled piezo-electric dome speaker, for example the Audax FTD-3P. Such speakers are prone to non-linearities and even harmonic distortion, for example due to the outward excursion of the speaker diaphragm being less than the inwards excursion for a given voltage. A single diaphragm speaker also suffers from the drawback that the gas acts as a non-linear spring, providing a stiffness which varies with volume.